The present invention relates to an opto-magnetic pick-up device for use in an information recording, reproducing and erasing apparatus which utilizes an opto-magnetic effect.
The opto-magnetic recording and reproducing effect means that a writing light beam is irradiated onto a magnetic recording medium having an easy axis in the vertical direction to the surface of the medium to reverse its magnetization thereby performing information recording, and a reading light beam is irradiated onto the medium to detect the difference in rotation of the polarization surface caused by the direction of magnetization due to the Kerr effect thereby reproducing the recorded information.
FIG. 1 shows a fundamental construction of a conventional optical magnetic pick-up device. A light emanated from a laser, unit 1 such as a He-Ne laser or a semiconductor laser is formed into a linearly polarized light by a polarizer 2 and incident onto a magnetic recording medium 7 through a beam splitter 3 and an objective lens 4. The light reflected by the medium 7 again passes through the lens 4, is reflected by the beam splitter 3 and incident onto a light detector 6 through an analyzer 5 thereby detecting the recorded information.
The magnetic recording medium 7 comprises a substrate 7a and a magnetic medium 7b which is formed on the substrate 7a by vapor deposition or sputtering magnetic medium material on the substrate 7a. As a material for the substrate 7a, glass, PMMA, polycarbonate and the like can be used. These materials have birefringence characteristics. For example, double refraction of glass and acryle is less than 10 nm and that of polycarbonate is an order of few tens .about.100 nm.
As shown in FIG. 3a, the light incident on the recording medium 7 is a linearly polarized light which oscillates only in a certain direction (this case, X axis direction). When such a linearly polarized light is incident onto the recording medium 7 which is magnetized in the direction vertical to its surface, the light reflected by the recording medium 7 has a polarization plane rotated by .theta..sub.K due to the Kerr effect as shown in FIG. 3b. That is, the polarization plane of the reflected light is rotated by .+-..theta..sub.K in accordance with the magnetizing direction and thus this rotation can be converted into the light strength by the analyzer 5 thereby obtaining reproduced signals.
While, if the substrate has a double refraction property the reflected light becomes an elliptically polarized light as shown in FIG. 3c without rotating as the linearly polarized light. A further factor causing elliptic polarization is a Kerr ellipse other than a Kerr rotation due to the magnetic Kerr effect.
Provided that the incident and reflected lights are polarized, the x component of the reflected light is .gamma..sub.xx, and the y component is .gamma..sub.xy, and the following relations are found. EQU .gamma..sub.xx =.vertline..gamma..sub.xx .vertline..sub.exp (i.phi..sub.x)(1) EQU .gamma..sub.xy =.vertline..gamma..sub.xy .vertline..sub.exp (i.phi..sub.y)(2) EQU If tan.alpha.=.vertline..gamma..sub.xy .vertline./.vertline..gamma..sub.xx .vertline. (3)
an angle of Kerr rotation .theta..sub.K, an angle of ellipticity .gamma..sub.K and a reflection factor R may be represented by following equations: EQU tan 2.theta..sub.K =tan2.alpha.cos(.phi..sub.y -.phi..sub.x)(4) EQU sin 2.gamma..sub.K =sin2.alpha.sin(.phi..sub.y -.phi..sub.x)(5) EQU R=.vertline..gamma..sub.xx .vertline..sup.2 +.vertline..gamma..sub.xy .vertline..sup.2 ( 6)
Where the term.phi..sub.y -.phi..sub.x is a phase difference caused by the Kerr effect. This phase difference becomes large when, as shown in FIG. 2, an enhancement means is used for the recording medium 7. That is, as shown in FIG. 2, a dielectric layer is provided between the substrate and the magnetic medium in order to increase the angle of Kerr rotation .theta..sub.K by the enhancement means. In this case, the phase difference .phi..sub.y -.phi..sub.x often becomes large due to the thickness of the recording medium.
Large double refraction of the substrate corresponds to a large phase difference. If the double refraction is 100 nm for the light with wavelength of 830 nm, the phase difference thereof is about 43.degree..
If the phase difference is large, .theta..sub.K becomes small and .gamma..sub.K becomes large in accordance with the equations (4) and (5). A S/N ratio at reproduction may be generally represented by a following equation. ##EQU1##
As the phase difference is large, the S/N ratio becomes deteriorated in accordance with the equation (7).
For example, if the phase difference is 0.degree. and 45.degree., .theta..sub.K is decreased to 0.7, even taking only the term sin .theta..sub.K in the equation (7) into consideration, the S/N ratio is described by about 3 dB.